This study was made to optimize treating conditions of eggs for producing gynogenetic diploids as well as to determine optimal methyltestosterone concentration for sex reversal of fry from genetic females to functional males in marbled sole Limanda yokohamae. Cold (-2.0 ~ -0.8°C for 60 min duration), heat (30°C for 5 min duration) and hydrostatic pressure treatments (700 kg/cm2 for 6 min duration) initiated 5 min after fertilization with genetically inactivated spermatozoa which had been irradiated with UV rays, gave relatively high frequencies of viable progeny. The resultant fish exhibited a heterozygous genotype which occurred by gene-centromere recombination and two unrecombinant homozygous genotypes at PGM* or IDHP* isozyme-coding locus, indicative of duplication of maternal chromosomes by inhibition of the second polar body extrustion. No viable fry was produced from the eggs subjected to cold shock at 165 to 205 min after fertilization. While, a small number of normal fish appeared in the groups treated with hydrostatic pressure at 175 to 205 min after fertilization. Isozyme analyses suggest presence of the complete homozygous gynogenetic diploids which were produced by inhibition of the first cleavage in these surviving fish. In normal diploid 14-month-old fish, females showed significantly better growth than males. However, gynogenetic diploids were significantly smaller than normal diploids. All the gynogenetic diploids examined were female. Immersion of the gynogenetic fry in 1, 5, 10 and 20 μg/ℓ of methyltestosterone between 21 and 80 days after hatching induced 53, 60, 73 and 80 percent of sex reversed males, respectively.